Viscous polybutenes of about 200 to about 3000 M.sub.n have viscosities in the range of about 4 to about 5500 centistokes at 100.degree. C. Such polybutenes are commercially available from polymerization of refinery butenes; isobutylene, cis-butene-2 and butene-1 are generally present with butane in a C.sub.4 fraction. Commercially since about 1940, such C.sub.4 fractions with or without added isobutylene, or isobutylene rich concentrates have been polymerized in the presence of Friedel-Crafts catalyst. The wide range in viscosity, and in molecular weight depends, as is known, on polymerization temperature, to a lesser extent on catalyst and its concentration, and on the olefin content of the feed. The viscous polybutenes are essentially water white and thermally decompose with no residue at temperature above 275.degree. C and have some use applications in engine oils as anti-scuff agents and viscosity index improvers and in fuels for internal combustion engines to reduce or suppress deposits in the fuel induction systems.
The viscous polybutenes have also found use as components of caulking compounds, adhesives and electric-cable insulating oils. However, the greatest use of the viscous polybutenes is as a raw material in the manufacture of addition agents for fuels and gasoline because the viscous polybutenes are reactive olefins and provide branched-chain alkyl structure in derivatives enhancing their solubility in petroleum products such as lubricant oils, fuels and refinery streams. The derivatives of most interest in the past 15 years are from the polybutenyl-substituted intramolecular anhydrides of aliphatic dicarboxylic acids such as succinic anhydride. The polybutenyl-substituted saturated aliphatic anhydrides have been used per se, or as diesters, amides, imides, amidines, imidines, and neutral or overbased basic metal salts as addition agents in petroleum products. The addition agents from polybutenes of M.sub.n below 500 are mainly used in fuels; for example in gasoline to inhibit rusting, carburetor deposits, and carburetor icing and in diesel fuels to inhibit rust, corrosion and smoke, and in motor oils and industrial oils as rust and wear inhibitors.
The addition agents from polybutenes of 500 to about 3000 M.sub.n have found extensive use as detergent-dispersants in motor oils and lesser use as carburetor detergents in gasoline, heat exchanger antifoulants in refinery streams, rust and corrosion inhibitors in surface coatings and as emulsifiers and demulsifiers.
The viscous polybutenes are complex mixtures of polymers, copolymers and interpolymers of isobutylene, cis-butene-2 and butene-1. The nature and relative amounts of the butene monomers involved in the polymerization leading to a particular M.sub.n polybutene are not indicative of the resulting polymer product because extensive isomerization occurs during polymerization. The viscous polybutenes, although largely monoolefins, may contain 0 to 20% isoparaffins. The unsaturation in the viscous polybutene molecules is predominantly in a terminal or near terminal group which, as later illustrated, are of the trisubstituted or vinylidene type. The non-olefinic chain portion of the polybutene molecules is composed of normal butyl and isobutyl monomer units and hence is a long and branched alkyl chain. Such long, branched alkyl chain of the lighter (below 500 M.sub.n) polybutenes contain relatively greater amounts of normal butyl units and lesser amounts of isobutyl units. The heavier (500-3000 M.sub.n) polybutenes contain relatively greater amounts of isobutyl units and lesser amounts of normal butyl units which are concentrated near the end of the long, branched alkyl chain. For example, the structures of a polydisperse polybutene of about 900 M.sub.n made with AlCl.sub.3 catalyst have in part been identified through the use of infrared spectroscopy (calibrated by NMR) and permanganate cleavage. The principal olefin structures identified are shown below ##STR1## wherein R is the long, branched alkyl chain, R' is mainly methyl and the ratio of iso-C.sub.4 to n-C.sub.4 is about 3:1.
With respect to polybutene addition reactivity with unsaturated intramolecular anhydrides, it is believed, that the olefinic terminal groups in the three structures shown above are in the decreasing reactivity order of III, I and II. In the uncatalyzed addition reaction, which is an equilibrium reaction, some of the slower reacting molecular species remain unreacted and with the isoparaffinic polymer species (0-20% of the total polymer product) which do not react at all, the desired polybutenyl-substituted saturated anhydride product is usually obtained in 50-75 mole % yields.
Such addition reaction between the viscous polybutene and intramolecular anhydride of unsaturated aliphatic dicarboxylic acid can typically use any one of maleic anhydride, citraconic anhydride, itaconic anhydride, ethyl maleic anhydride, halo (e.g., chloro-) maleic anhydride, and the like according to U.S. Pat. Nos. 2,628,942 and 2,634,256 among others. The addition reactions are, in general, conducted at temperatures in the range of 150.degree. to 300.degree. C using polybutene to anhydride molar ratios of reactants in the range of 1.0:0.8-15, generally 1.0:1.05-1.15. In addition to the non-reaction of some olefinic species of polybutene and isoparaffinic entities thereof amounting to a total of up to 40-50% of the polybutene charged, there is also a problem with respect to thermal decomposition and polymerization of the unsaturated anhydride reactant at temperatures upward from 150.degree. C.
Thermal decomposition at temperatures upward from 150.degree. C of unsaturated aliphatic dicarboxylic acids and their anhydrides (e.g. maleic and its anhydride) has been known and is reported, for example in U.S. Pat. No. 3,476,774 which gives earlier documentation sources thereof. Such thermal decomposition is accompanied by evolution of water vapor and oxides of carbon, in a closed reaction vessel, is accompanied by an increase in internal pressure. Under some observed conditions the thermal decomposition can be so substantially instantaneous as to be explosive. In the absence of explosive thermal decomposition a carbon-containing residue is also formed in addition to water vapor and oxides of carbon. Anhydrides can react with the water to form the dicarboxylic acids and then isomerize to the trans form (which is insoluble in the system) or to polymerize. Such thermal decomposition and attendant isomerization or polymerization of the unsaturated anhydride reactant has been observed as occurring during its addition reaction with polymeric olefins, e.g. polybutenes and others, in a closed reaction vessel. There is the increase of internal pressure by involved water vapor and oxides of carbon (mainly CO.sub.2) but the attendant carbon-containing residue varies in nature from somewhat granular when the decomposition is only slight to a tarry material mainly adhering to internal surfaces of the reaction vessel when the decomposition is more extensive but well below explosive magnitude. The granular type residue amounts to from about 0.1 to about 0.3 weight percent of the total charge is generally dispersed in the alkenyl-substituted saturated anhydride addition compound product diluted with unreacted components of the olefin polymer, and is readily separated therefrom by filtration. However, the tarry residual, product, which for the most part fouls the internals of the reaction vessel can be as high as 2-3 weight percent of the total charge. The tarry residual material not adhering to reactor internals fouls the filter and interferes with filtration of the desired reaction product. Both types of residue are undesirable because of the above noted fouling characteristics and because their formation results in yield reduction of the desired alkenyl-substituted anhydride addition product.
Generally after a commercial anhydride-alkene reaction, unreacted anhydride is stripped from the reaction product and discarded because it is impure due to the undesired side reactions and would result in low yields if used for further reaction. Purification of the recovered anhydride is generally economically unattractive. The wasted anhydride not only adds to operating costs but also presents a disposal problem.
U.S. Pat. No. 3,202,679 does describe a process for preparing alkenyl succinic anhydrides whereby unreacted maleic anhydride is recovered and recycled. This reference explains that water derived from the decomposition of maleic anhydride hydrolyzes a portion of the maleic anhydride to maleic acid. By using certain process conditions, the maleic acid is isomerized to fumaric acid. Unreacted maleic anhydride is distilled off for recycle and the reaction product is diluted with oil and then filtered to remove insoluble impurities such as fumaric acid. This process does not reduce maleic anhydride decomposition, but only prevents some of the decomposition products from contaminating the recovered maleic anhydride. Another drawback of this process is the additional processing step of separating the fumaric acid from the alkenyl succinic anhydride product.
The improved process disclosed herein produce high yields of alkenyl substituted anhydrides while substantially diminishing the amount of anhydride waste. Decomposition of anhydride and formation of undesirable by-products is minimized, hence unreacted, recovered anhydride and reaction product contamination is also minimized without additional processing steps.
The term "tar and side product suppressing additive" shall be used to describe a chemical compound which inhibits the formation of tarry residual matter and undesirable reaction side products of maleic anhydride and/or increases yield in the reaction of maleic anhydride with propene or butene polymers having a molecular weight from about 200 to about 3000 at a temperature from about 150.degree. C to about 300.degree. C to form a substituted succinic anhydride when said additive is present during the reaction between the polymer and the anhydride at a concentration of 5 to 200ppm based on polymer.